Method of and apparatus for aerotriangulation with convergent photography



May 7, 1957 J. T. PENNlNGToN 2,7951

METHOD OF AND APPARATUS FOR AEROTRIANGULATION WITH CONVERGENT PHOTOGRAPHY ATTDRN EYS May 7, 1957 J. T. PENNINGTON 2,791,151

, METHOD OF AND APPARATUS FOR AEROTRIANGULATION WITH CONVERGENT PHOTOGRAPHY Filed May 6, 1955 15 Sheets-Sheet 2 INVENTOR. John Tfennz BW ATTD RN EYS J. T. PENNINGTON 2,791,151

May 7, 1957 METHOD oF AND APPARATUS FOR AEROTRIANGULATION WITH CONVERGENT PHOTOGRAPHY Filed May 6, 1953 -15 Sheets-Sheet 5 May 7, 1957 J. T. PENNINGTON 2,791,151

METHOD OF AND APPARATUS FOR AEROTRIANGULATION WITH CONVERGENT PHOTOGRAPHY Filed May 6. 1953 15 Sheets-Sheet 4 region ATTD R N EYJ` May 7, 1957 J. T. PENNINGTON 2,791,151

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May 7, 1957 J. T. PENNINGTON 2,791,151

METHOD OF` AND APPARATUS FOR AEROTRIANGULATION WITH CONVERGENT PHOTOGRAPHY Filed May 6, 1953 15 Sheets-Sheet 7 ATTURN EYS May' 7, 1957 J. T. PENNINGTON 2,791,151

METHOD OP AND APPARATUS POR AEROTRIANGULATION WITH CONVRRGENT PHOTOGRAPHY Filed May 6, 1953 15 Sheets-Sheet 8 Q/ INVElvToR. 74 5 Jaim Tfemzzryimz ATTDRN EY5 May 7, 1957 J. T. PENNINGTON METHOD oF AND APPARATUS FOR AEROTRIANGULATIO I WITH CONVERGENT PHOTOGRAPHY 13 Sheets-Sheet 9 Filed May 6. 1953 mb \%N mk @Si a s 0 n .MN @//7 n Sm, m10 M m m Q\ m-Z A .@Hwnw w n ww. wm. 1 my? A N@ MH @1 nA Nm@ um. .bm .mwwr .N6 WMU LN QMMN @N mwN 1m if WSN N. .NQN w J W 4 HA A... .W -W%\F 1 May 7, 1957 J. T. PENNINGTON METHOD OF AND APPARATUS FOR AEROTRIANGULATION WITH CONVERGENT PHOTOGRAPHY 15 Sheets-Sheet 10 Filed May 6, 1953 INVENToR. John 7.' Henning i072 BY f 7/ n/ r M4K 7 www www\ www www www wwwwwwmw www NJmnrwN LHN wwv www o www wrd www( www T. www www 4 -..-WHHHVHMHMHHW@ v J; .t LNL www ww www www @www www www NN. www w A@ w .V @.--.www ww, www www@ ww www l! MIMI l@ .v www r mm\ M `.I NNW kvwm .www QN wNN NNW ATTD R N EYJ May 7, 1957 J. T. PENNINGTON 2,791,151

METHOD OF AND APPARATUS FOR AEROTRIANGULATION WITH coNvERGENT PHOTOGRAPHY 13 Sheets-Sheet 11 Filed May 6, 1953 Ignis sw n ws 29. Jom .7.7297272 2M BY ATTDRNEYJ May 7, 1957 J. T. PENNINGTON 2,791,151

METHOD OFAND APPARATUS FOR AEROTRIANGULATION WITH CONVERGENT PHOTOGRAPHY Filed May 6, 1953 l5 Sheets-Sheet 12 JNVENToR. Pennz'ntgorz llllllll- IVI-Ill!! ATTEI RNEYS May 7, 1957 .LTPENNINGTON I 2,791,151

METHOD 0F AND APPARATUS FOR AEROTRIANGULATION WITH CONVERGENT PHOTOGRAPHY Filed may 6, 195e 1s sheets-sheet 1s I Mobel. IF -zn IL Monsv. 2F -an I F52g-45- PROCEDURE B 2R Mons. IF-zn STEP 1 STREOPONTOMETER MODEL ZFZB LORIENT Mona nF-zn To RouNvcoumoL STEP n 2. RECORD SINON AND ELEVATION 0F PASS LREPLACE IF WITH 2F Pomrs. 2. nom omeumnou oF 2R PROJECTOR Aun ADJUST 2F PRoJEcron Rwmvs To 2n Psoascron STEP m LREPLACE 2R WITH 3R 2.HOLD ORIENTATION 0F 2F PROJECTOR AND ADJUST 3R PROJECTOR RELATIVE TO 2F PROJECTOR 3. SCALE T0 PASS POINTS ESTABLISHED IN THE FIRST MODEL IF-ZR 4. ORIENT REMAINING MODELS AS OUTLINE-ID ABOVE IN1/nvm. Ja/m ZPezznznggfon United States Patent METHOD F AND APPARATUS' FOR AER'I'RIi ANGULATION WITH CONVERGENT PHOTOG- RAPHY John T. Pennington; Alexandria, Va.

Application May 6, 1953, Serial No. 353,454

14 Claims. (Cl. 88-24) (Granted vunder Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without payment to me of any royalty thereon.

The present invention relates to improvements in existingy methods and equipment employed formakingaerial surveys, in which there is utilized a plurality of imageprojecting or carrying cameras for survey and reproduction. The invention relates particularlyA to topographic surveying and mapping by means of stereophotographs of any description, that is to say, by means of stereo photographs of anysize, focal length, and direction of exposure obtained by means of any known system, such as projection. of ray intersection, stereoscopys or any other means known to the art. However, the presentin-V vention achieves the surveying in a substantially simpler manner than has been employed heretofore, and utilizes high-precision? equipmentl of' simpledesign and construction which enables the attainment of high accuracy in results substantially more conveniently and expeditiously than has been possible heretofore.

In the surveying and reproduction ofterrain, vthat is, areas of the earths surface, to which art the present invention relates as has been vindicated above,'prior practices include taking photographs from an aircraft at certain distances apart, whichphotographs partly overlap, so thateach successive pair of lms or platescom stitutes a. steroscopic photograph oftheterritorial area photographed on the two plates. These stereoscopic picturesare placed in a stereoscope and the rays are caused to. intersect. in' pairs in such a mannerrthat the'totality or integration of the intersections develops an optical model olthe photographed territorialL area, from-which model a map can be drawn, or a relief model of the section of the territory can be produced. Thismethodvr encounters substantial difiiculties because the pictures usually' are taken at high elevations which practically amount to in# finite distance, and in the reproduction, that-lis, theI formation of the optical model, the complementary'rays must1be causedto intersect over the drawingfor survey.

table. The'present invention overcomes the above-re-y ferred to diculties to a very substantial extent,y as will become apparent hereinafter.

In the devices heretofore known and intended for the same puropse as the present invention, the optical laws holding true in geometric optics and the-linearratio' of magnications as used for single projections, for'example, inthe magic lantern and moving picture projection, have been ytransferred to doubleprojection; and this'has'resulted rin serious inconvenience in using the known devices, which inconveniencethepresent invention elmi nates.

- Heretofore, in order that the necessary overlap of' the pictures Was arrived at wit-hout the limit of solution of the grain of the plates being exceeded, two methods' have been employed, namely:

(1) ,Optical means, suchas mirrors, prisms, and inter; mediate optic arrangements, havebeen' interposed 's'ol as 2,791,151 Patented May 7, 1957 Farice tord'eect'thee ray, which brought the plates into an unnaturalposition. In this way the cameras are prevented from interfering with each other, but otherwise the set so: obtained. is heavy and expensive and liable to substantial error; the intermediate members cause sensible lossesof light and the external orientation of the plates isrveryinconvenient and tedious.

(2.) The distance' between the two cameras (projection base): has been so large that it was practically impossible tovh'andle them'- The large projection base also caused allthe rest of the apparatus and the separation of the planes of sharp projection to be inadmissibly large. The projections were. so dark that very powerful and strongly heatinglarnpsv were necessarily used which produced conditions destructive ofA accuracy. Another serious inconvenience'iof the'. largeA projectionV base isv that the grain of the plates is also magnified whereby the accuracy of measurement isisubstantially impaired.

Defective accuracy and serious uneconomical factors in manufacture and operation constitute the main reasons for'fthe only occasional use of the known expensive apparatus.. and' awkward procedures attending the use of such apparatus' and for the failure of topographical surveyingby means of stereoscopic pictures taken from aircraft an'dfA the. making of optical models therefrom to emergefronti-the` experimental stage. Efforts made previously' to-the present invention to obviate the previously encounterediobjectionsand diiculties have been at best onlypartiallyfsuccessful and leave much toA be desired in the expeditious and economical attainmentv of the desired and'. neededvk objectives.

Aslias been noted above, the present invention obviatesr the previously encountered diiiculties in this art. Generally speaking, itis the principal object of the present invention to overcome the disadvantages encountered? in the construction and use of the previously available equipment, and to provide appartus, particularl'y,; dual aerial camera equipment' and a double projector,` for stereoscopic photographing and projecting whichoperates with the greatest accuracy, which is relatively inexpensive to manufacture, and which is economical and easy to operate.

MoreA particularly, a further object of the present invention isV toi provide apparatus and a procedure for topographicv mapping from aerial photographs, in which a}; paratus` theY photographs utilized are taken from a twinifcam'eraV installation of simple construction which obviates complex` and heavy prior constructions in which, asfhas beenl a requisite in previous equipment, the angle of tilt of the camera installation is required to be known tofavhigh degree of accuracy, the onlyr basic requirement in carrying out the present invention being that the camera installation: employed will produce sufficient overlap between thei photographs taken by each of the cameras to produce astereoscopic image or model through the useeof. improved equipment for obtaining such a model, which equipment is encompassed within the scope and objectsy of the present invention.

Arr-'further object of the invention in line with the foregoing,Y is to provide an apparatus whereby conventional double or'twin projector instrumentalities may be utilized in therobtaining of .stereoscopic images or'models from a simplified twin. camera installation, thereby eliminatingfthe.requirement for a high-precision, mechanically adjusted-.twin projector instrument heretofore required foraccurately reproducing the exact position of the twin camera installation at the moment of exposure of the photographic film or plate.

A still'f further objectfof-therinvention is to provide a method for bridging control between convergent oblique photographs of a number of observation stations, which can be applied to the use of anyinstrumentiutilizingonly two projectors, thereby eliminating any necessity of providing a large bank of projectors.

A still further object of the invention is to provide a method for bridging control utilizing the overlap cornmon to succeeding pairs of stereoscopic photographs or models.

Further objects of the invention will become apparent as the description proceeds, and the features of novelty will be pointed out in particularity in the appended claims.

In connection with the foregoing objects, it may be noted that prior to the present invention, it has been the usual practice in stereophotogrammetric mapping operations to utilize serial strip vertical photography in which succeeding photographs in the serial strip overlap Veach other by approximately sixty percent.' However, instead of vertical photographs, alternate vertical and oblique exposures have been used to a limited extent to take. advantage of the increased area covered through the overlapping pair, which increased area may be approximately twice that obtained with vertical serial strip photography, but even with the use of alternate vertical and oblique photographs, there is insutiicient overlap between succeeding stereoscopic models to effect a tie between such models as would be necessary to bridge between widely spaced ground control points. More recently, it has been proposed that serial strip convergent oblique photographs taken with a twin camera installation of very `complex construction for utilization in projectors of great complexities be used, but the cost of these machines has made their use largely prohibitive. The present invention is intended to replace such complicated and costly equipment as has been noted above asone of the objects of the present invention.

Before entering into a detailed description and explanation of the present invention, it may be noted that `the invention makes use of a basic principle that any object when photographed from different points will appear to be placed differently with respect to other objects shown` in the photographs; and in the description of the principle and operation of the invention which will be set forth hereinafter, it will be assumed that aerial photographs taken from different points are employed in pairs, the photographs having adequate overlapping areas, each photograph of the rst pair showing readily identifiable objects the exact location in space ,of which are known, or which can be ascertained in any known way, as well as having other known or readily obtainable characteristics such as height or other dimensions, physical appearance and the like, which objects serve as reference points, each succeeding pair of photographs containing these same objects or other additional objects, the exact locations in space of which are desired, and which, being in the area of overlap of the successive photographs, serve as passV points. Such a series of pairs of photographs are known as serial strip convergent photographs. It is evidentrthat because of variations in ground elevations and, in the case of aerial photographs, because of tilting of the cameras because of unsteady ight of the airplane engaged in the photographic operations, as well as from other causes, the photographs themselves will give a distorted view of the terrain, and such photographs when placed in juxtaposition for stereoscopic viewing, are of different scales. All of these factors necessitate corrections for such distortions and also a coordination of the series of pictures using the one containing as reference points the aforesaid objects of known spatial position as a reference for the remaining pairs of photographs. The procedure perfecting this coordination of the photographs is referred to as bridging control. As has been pointed out above,.the present invention provides an improved method of effecting this bridging control and a simplified apparatus for carrying out the improved method.

Reference now may-be made to the accompanying drawings, in which:

Fig, 1 isa diagrammatic showing of the taking of convergent serial strip photographs, with a wide angle twin camera apparatus, of a series of stations along a line of flight, the cameras photographing overlapping areas of terrain adjacent to the stations;

Fig. 2 is a diagrammatic assembly view of an improved apparatus forming a part of the present invention and used in the practice thereof;

Fig. 2a is a perspective view of an improved photogrammetric stereopontometer which may be used in the practice of this invention;

Fig. 3 is a plan view 0f the photogrammetrlc stereopontometer of Fig. 2a;

Fig. 4 is a plan view of the main supporting frame of this equipment and its carriages;

Fig. 5 is a side elevation, parts being shown in section, of the main supporting frame and carriages shown in Fig. 4;

Fig. 6 is a sectional view of the main supporting frame taken along line 6 6 of Fig. 4;

Fig. 7 is a sectional View ot the left carriage for the main supporting frame taken along line 7 7 of Fig. 4;

Fig. 8 is an enlarged fragmentary front elevation of the stereopontometer, showing the platen and binocular mounting brackets;

Fig. 9 is a vertical transverse sectional view of the stereopontometer taken along line 9--9 of Figs. 2a and 8;

Fig. l0 is a vertical transverse sectional view of the stereopontometer taken along line 10--10 of Figs. 2a and 8;

Fig. 11 is a plan view of the stereoscope supporting frame;

Fig. 12 is a front elevation of the stereoscope supporting frame;

Fig. 13 is a vertical sectional view of the stereoscope supporting frame, taken along line 13--13 of Fig. 1l;

Fig. 14 is a perspective View of one of the brackets used for supporting the platen units and binoculars;

Fig. `15 is a perspective view of a nut utilized for spacing the platen units;

Fig. 16 is a plan View of the platen spacing scale, also shown in perspective in Fig. 2a;

Fig. 17 is a front elevation of the platen spacing scale;

Fig. 18 is a transverse sectional view of the platen spacing scale, support and guide taken along line 18-18 of Fig. 16;

Fig. 19 is a longitudinal sectional view of the platen spacing scale, support, and guide, taken along line 19-19 of Fig. 16;

Fig. 20 is a perspective view of the scale support and guide;

Fig. 21 is a detailed perspective view of one end of the main supporting frame, showing the ball bearing supporting rod plates removed;

Fig. 22 is a perspective view of one end of the stercoscope supporting frame;

Fig. 23 is an end elevation of one of the main supporting frame carriages;

Fig. 24 is a plan view of the main supporting frame carriage shown in Fig. 23;

Fig. 25 is a detailed sectional View of the main supporting frame carriage, the section being taken along line 25-25 of Figs. 2a and 4;

Fig. 26 is an enlarged detailed sectional view of the main supporting frame carriage roller mounting eccentrics, the section being taken along line 26-26 of Fig. 25;

Fig. 27 is a detailed sectional View of the main supporting frame carriage brake, the section being taken along line 27-27 of Fig. 25;

Fig. 28 is a detailed sectional view of the platen bracket roller mounting eccentrics, the section being taken along line 28-28 of Fig. 8;

Fig. 29 is a horizontal sectional view of the platen bracket roller mounting eccentrics, the section being taken along line 29-29 of Fig. 28;

Fig. 30 is .a detailed vertical sectional view of the ,bracket retaining guide,` lthersection being ,taken along, line ASAI-#Stfof Eigr29.; k

Fig. 31 is an end elevation ofthe right main Vsupportlingfgframe. carriage show-n in Iperspective oniy theright side oli-Fig. 2a;

Fig. 32 isaplan view of themainsupporting carriage of Fig. 31;

Fig. 33 -is a sectional viewof themain supporting frame carriage, the .section beingtaken alongjline 33'-33 of FigrZa;

Fig. 34 isa detailed sectional view yofthe main supportingframe carriage roller mounting eccentrics, the sections being ttaken along line34-34 of Fig. 33.;

Fig. 35 is adetailed end viewfof the-tubular` track mounting;

Fig. 36'is aplan view-of thetubular track mounting shownvin Fig. 35;

Fig..37is a -vertical detailed sectional view of the 'tubular4 track amounting,A thesection being taken along line 37'-37 'of Fig..35

Fig.38 isa side'elevation, parts being. broken away :and-fshownin section; disclosingdetailsfof. one eyepiece;

Fig. 39'isA a .front elevation, parts being broken @away zand 4in section, disclosing details of one eyepiece;

Fig. 40 is a front elevation of one of the* platen jhousings;

Fig. tl .isa side elevation of one of the platenfunits Zhaving; parts being lbroken :away and -in section; disclosing -details of one ofthe platen units; l

Fig. 42fis a diagrammatic view showingthe production fofstereoscopic models from the convergentserial strip photographs takenin accordancefwith Figl;

Fig. 43A is a diagrammatic view showingtherst-step fin the productionof a stereoscopic model utilizinglthe :improved method of bridging control of the present'invention; and

Fig. 44 is a diagrammatic view illustrating the second land thirdsteps'in the production of stereoscopic models lin accordance with the present'invention,jutilizing the projectors and viewing equipment illustratedin Fig. 2, the procedural steps being Shown with the;assistancc'of self-explanatory legends, Figs. 42, 43, and'44-being-in tended to be consideredconjointly with reference-to both Fig. 2y and to Fig. l, and particularly with reference to Fig. l.`

Before proceeding with a detailed description ofthe accompanying drawings and the. improvements ofthe present invention illustrated thereby, it may be pointed out` that within `certain limits, a stereoscopic Viewy or model of any subject maybe obtainedby fusingV two pictures of the subject taken fromdilerentviewpoints solthateach eye ywill see but one view.- However, this view ormodel is generally not aftrue-to-'scale reproductionof the subject. Each'picturc'is a 'perspective' View of the subject, kthe camera lens being the center of rperspective.4k In order to have a true perspective view, the taking lens must be without distortion, or the ypicture must be viewedthrough the takinglens or a'lens exactly like it as to shape, position, and location'. Assumingftwo -true perspective views, to obtain a correct stereoscopic model the two views-must befused with exactly the same angular relationship as occurred` in Anature when `the pictures were taken. Practically, this means the taking positions and orientations of the two picturesmust'berecovered For aerial photographs this is `not possible'by any'system of direct measurement. l

A true stereoscopic model can .be-formed from two correct perspective views if the pictures can beprojected fromrthe same. orsimilar cameras, so that-all rays from correspondingupicture pointsv intersect in space; Todo this with certainty and to obtain. a model that 'can be used for mapping,k the. following requirements must be fulfilled:

1V. AA .stable-original negative, on glass A plate or.n good lijm.

frame 2.. Projection from. an` `exact duplicate,4` of .the taking ycameraorfrom an. exact ,reduction .or enlargement of the taking camera; inthe latter case the same reduction 0r enlargement-of the photograph must be made.

3. Aprecisemethod of orienting the two projections, both relatively and absolutely, relative orientation` being defined as orientation-of one projector with reference. to another toproduce the relative .relationship vof the taking cameras. Two .adjacent projectors are relatively oriented when all rays from .one intersect corresponding rays from the other. Absolute orientation is denedas orientation of two or more projectors to give.astereoscopicmodel having correct scale, andhorizontalization. Thismatter oforientation, bothv relative. and absolute, of the pro.- j'ectors employed. in the present procedure will be ,-referre to again hereinafter. Y

4.y A-method of'viewing the .double projection. without changing this orientation..

5; A method, of measuring:.thestereoscopic model. v

In, carrying outthepresent Vprocedure photographs are employed which have been takenin the mannerhereinafter described vby the use of viilm cameras complying with Ythe requirementsof a precision camera. Thesephotographs may be reduced in size andV made intopositives, which may be either transparencies or-opaque, Ydepending ony theY type of projector to be employed, so that, they may be used in projectors of small size. The projectors may be visualized` as replicas of the .taking.camera s, so placed along` a horizontal bar as to be inthe same relative positions occupied by the camera when each picture was taken. Rays from adjacent projectors intersect to form images inthe samerelativepositions they occupied `on the terrain. These'images are viewed stereoscopically, measurements are made inthe stereoscopic model, and the ydetail inthe model is compiled into a conventional map onpaper.

In carrying out the. present invention it will be understood that a pair of cameras is mounted on an aircraft and operatively connected for simultaneousactuation so as lto produce photographs of terrain to be mapped, taken simultaneously by the twocameras of the pair, lone of which cameras being directed forwardly of the aircraft and in the direction of the line of ilight of the` aircraft, the other camera ofthe pair being directed inthe opposite` direction, that is, rearwardly of the aircraft and ofthe line of flight, the former` camera taking a` forward oblique View of an area of theterrain being mapped, vthe latter camera taking ak rear oblique view of the ground surface within range or view of this latter camera. AS the. ght ofthe aircraft progresses over the. said terrain, a seriesof such exposures will be made, each exposure being a forwardoblique view of thel terrain andY a rear oblique view thereof, the exposures. being madev at such time intervals that the rear obliqueviewrtaken atthe instant of exposure of the plates or films in the two cameras will overlap to a substantial extent thelprecedinglymade forward exposure, so that the` successive pictures always will show a substantial overlapping area' of the terrainwhich area is common to thefforward oblique view taken by previous exposure andthe rear oblique view taken by the succeeding exposure. The sequential exposures are taken at such time intervals that will be necessary to cover the entire terrain to be mapped While preserving a substantial overlapping area commonV tothe forward oblique view of one exposure and the rear oblique view of the immediately succeeding exposure. It willjbe understood that the operation of the camera shutters is timed to operate automatically at constant'selected distance of flight of the aircraft and also so that the angle of directional slope from the respective cameras for each exposure will Vbe the same; and the distance between the succeedingpoints of exposure has to be s'o selected that the resulting series of yphotographswill have a common area'of overlap, that is, Vthe photographsof-each 'station have areas or amounts by'which one photograph over flaps the area covered by another, which amount of overlap is expressed customarily as percentage. The resulting exposures are developed, and positives made therefrom which are assembled in a pair of projectors which 'tive to the terrain being photographed, as well as variations ofits line of ight above the ground line. Since the production of true stereoscopic models depends upon y exact relative positioning of the projectors to duplicate the positions of the cameras at the instant of making the exposures, orienting corrections of the projectors must be made so that such variations in camera relationships may ,n

'be corrected and the overlapping areas of the succeeding photographs will be brought into true stereoscopic orienta- 'tion or relationship. The viewing device of the present `invention greatly facilitates the exact production of true `stereoscopic models.

As has been pointed out above, the present invention makes use of the basic principle that an object when photographed from different points will appear differently placed with respect to other objects also shown in the photographs, and is especially useful in preparing maps from data supplied by photographs taken from aircraft `in flight. Accordingly, in the description of the principle and operation of the invention, it will be assumed that aerial photographs taken from different locations are Aemployed in pairs, each photograph of the iirst pair showing objects to serve as reference points, the exact location Aof which in space are known or which can be ascertained in any known way, and each succeeding pair containing identifiable objects the exact location of which in space are desired and which are designated as pass points,

such a series of pairs of photographs being known to thc art as serial strip convergent photography. It is believed Vto be apparent that because of variations in ground elevations, and, in the case of aerial photography, tilting of the cameras by theunsteady ight of the airplane, and other phenomena, the photographs will give a distorted picture of the terrain, and such pictures also likely will be of diierent scales so that when* they are placed in juxtaposition for producing stereoscopic models, will require corrections for these distortions and a coordination of the series of pictures using the one containing objects of lknown spatial position as a reference for the remaining ypairs of photographs. The procedure perfecting this coordination of the photographs has become known to the art as bridging control, and, as has been pointed out above, it is for a new method of bridging control and an apparatus for carrying out this method that this invention is specifically directed.

In the past it has been the usual practice in stereophotogrammetric mapping procedures to utilize serial strip vertical photography in which succeeding photographs in `the. serial strip overlap each other by approximately 60%. However, alternate vertical and oblique exposures have -been employed to a limited extent to take advantage of the increased area covered through the overlapping pair,

whichin this case is approximately `twice that obtained Awith vertical serial strip photography, but even with this v.type of photography there is insuflcient overlap between succeeding photographs to elect a tie between stereoscopic .models produced from such photographs as would be nec- .essary to bridge between widely spaced ground control points More recently it has been proposed that serial strip convergent oblique photographs taken with a twin Ycamera installation be employed for mapping. This twin Icamera.n installation for taking photographs of the above meinst 'type which can be used in the usual projectors for pro ducing maps of great accuracy are expensive, due to the complexity of their construction.

Specific reference now may be made to Fig. 1 which, as has been stated above, shows diagrammatically a manner of taking convergent oblique photographs suitable for use in the present invention. In this view, the upper-horizontal line designates an imaginary line of flight and direction of ight above a ground line, which is indicated by the lower horizontal line, While the reference letters A, B, and C represent a series of consecutive points of camera exposures along said line of flight over the terrain to be mapped and which is to be covered by a series of pairs of convergent oblique photographs taken with a twin camera installation using wide angle aerial cameras having an angular coverage across the terrain being photographed. As illustrated in Fig. 1, the wide angle aerial cameras employed for making the exposures have an angular coverage across the format of the photograph of about 75 degrees, and each camera is indicated as being tipped respectively forward and backward along the line of ilight through an angle of approximately 20 degrees with re spect to the vertical. Thus both cameras simultaneously photograph an area extending from a point approximately 171/2 degrees to the rear of each point of exposure of the cameras to approximately 171/2 degrees forward of each of the said points, for a duplicate coverage of approximately 35 `degrees measured in the direction of flight at each exposure point. It is to be understood that these values are illustrative only and that other angular values would be equally satisfactory, the only basic requirement being that there is sufficient common overlap as will be hereinafter explained between the successive forward and rear exposures in each of the cameras in the twin camera installation. With the angular values for camera cover age and tilt given above, the spacing between successive camera exposure points could be as much as 11/2 times the flight altitude above the ground. The pairs of convergent oblique photographs taken from successive camera exposure points A, B, and C, Fig. l, include for each of the said exposure points a forward oblique view delineated by dotted lines as to the limits thereof and a rearward oblique view shown by solid lines. Therefore, from Fig. l. it will be seen that a forward oblique view taken from an exposure point, such as A, is overlapped by an amount X by a rear oblique view taken from the same exposure point, and that the forward oblique view taken from the first exposure point A is overlapped by the rear oblique 'view taken from a second exposure point B in an area of terrain common to these two views which overlapping or common area is indicated by the letter Y along the ground line; and the rearward oblique view from the next surf ceeding (third) exposure point C overlapping the forward oblique view from the preceding point B in a common area designated by the letter Y along the ground line, thereby forming a stereoscopic pair of photographs. Also, the forward oblique view from the iirst exposure point A is shownlas having an area portion common to and overlapped by the rearward oblique view from the third exposure station C in an area designated by the letter Z" along the ground line, this area Z being also common to the area in the forward and rearward oblique views from the second exposure point B, this area Z also forming a common area of overlap between the two stereoscopic pictures formed by the forward oblique view from the first exposure point A and the rear oblique view from the second exposure point B and the forward oblique view from point B and the rearward oblique view from the third exposure station C, from which stereoscopic pictures two stereoscopic models may be obtained and scaled as will be described hereinafter.

In the practicing of the present invention, developed negatives are obtained from the exposures made in accordance with Fig. 1, and positives are made from such developed negatives. Regardless of the number of exposure points required to cover a desired strip of terrain, the operation of the present invention contemplates the use ofy only two projector means which correspond to the twin kcameras.employed in taking-the original eX- posures. The general layout of the equipment is shown schematically in Fig. 2 ofthe drawings, this view indicating the two projection .devices as projector 1 and projector 2,- respectively, eachof which projects a single image of photographed terrain downwardly upon a pair of platenunitsof a binocular viewingdevice, also shown diagrammatically. on Fig. 2. This viewing device is mountedon la table support having a control sheet of drafting ypaper thereon yfor recording the data obtained from the projected photographic images. Each image s projected is composed of photographic compositions which are assembled as will be described hereinafter. Theabove projectors may be of any conventional type capable of receiving positive prints of the negatives of thexviews obtained in-accordance with Fig. l, therefore, no `detailed description thereof is believed necessary as theV constructiony and operation thereof are wellfknown in-.the art. Suce itv to say that these projectors are mounted on a supporting bar through a coupling which will` permit the projectors to be tipped in a forward or backward: manner, tilted to either side, `and raised or lowered vto a desired position for effecting proper orientation of stereoscopic photographic images projected thereby. In connection withsthe improved method of bridging `control forminga part of the present invention, it may be noted that thev stereopontometer is not absolutely necessary to the carrying out ofl the present improved method `of bridging control. However, a high order of precision in the orientation -of the forward obliquesrelaf tive tothe rearwardobliques is obtained by its use.

The description of the improved'bridging stereoscopic registering andi viewing devicel or stereopontometer ern-y ployed in connection with the present invention-will be more readily understood when its function lwith respect to the novel method of bridging control has been described. Accordingly, the method of bridging control utilizingthe said stereopontometer will first be described: From thev above-described photographic positives, made from negatives obtained in accordance with` Fig. l, pairs of stereoscopic photographs are made, each pair consisting of theV forward oblique photographic; positive of one exposure point and the rearward oblique positive of the next succeeding exposure point. In performing this method of bridgingy control, the forward oblique positives may` be placedsuccessively in projector 1 (Fig 2) and the corresponding rearward oblique: positives successively in projector 2 (Fig. 2) beginning with the forward obliquepositive `taken from exposure' point A and the rearward oblique positive-taken from exposurepoint B, which positives have been produced from the developed negatives resulting from the procedure of Fig.k l.

The forward-oblique photographic positive taken vfrom expo-sure point Av is positioned within/*projector 1, and the rearward oblique photographic positive from exposure point B is positioned within projector v2 and the projectors then are oriented by manipulation ofthe projectors to form a stereoscopic model of the area Y,' Fig. 1, which may be brought to true scale and proper orientation byrelative and absolute orientation of the two projectors. After thisv orientation is accomplished and the data to be obtained therefrom recordedonvthe control sheet, the forward oblique photograph ofV station A is removed from projector 1, and the forward oblique photograph of station B is mounted in this projector. Without disturbing the orientation of projector 2 housing. the rearward oblique photograph of station B, projector 1 is oriented so that its projectionof the area--X, Fig. l, is common to that in projector 2 and'is the exact size and shape on the plane of projection as the projection from the undisturbed projector 2.' This orientation willi recover thetilt, tip, swing, and height with respect to;

the projection planev ofthe forward oblique tof tite-fsta- ,tion ,.B, and maybe-greatlyfacilitated byv use of .thephotogrammetric stereopontometer hereinafter described; because the common area projected from the projectors will not overlap in their. projection, Vbut will be Shown ron opposite sides of the projected images as isk indicated in Fig. 4.4. Hence the stereopontometer is utilized to-literally bring the areasv common to the two projections together, permitting a visual determination as to when theV objects orpass points.discloscdin-theircommon area are of thesame-sizeand shape; Accordingly, proper orientation isaccomplished when projectory 1: is oriented with respectttoprojector 2 to show the stereoscopic imagesof the refereneefrnarks etched `on 'the imagereceivingscreens to be in coincidence with ythe photoimages in all portions of the common coverage. A rou'- tine trial and error-'procedure in manipulating the projector 1 can'be used to` effect vthis orientation. After this orientation'is made, Ythe rear oblique photograph of station :Bis removed l.from lprojectorV 2' and` the rear oblique photograph ofy station C is placed in this projector Zand the latter is oriented relativey to projector 1 to form av secondstereoscopic model with the forward oblique photo-- graph in-projector 1; In this orientation, projector 1 is undisturbed, hence the resultant stereoscopic model is-in correct absolute orientation with respect to the datum plane established 'by' the first stereoscopic model. The second stereoscopicv model is scaled by adjusting projector 2, housing the rearward oblique of station C, so that the velevation of-pointsin the -area Z, Fig, l, common to the rtirststereesoopic model formed by the forward oblique-of station A and thevrear `oblique of station B read the same as they did in the preceding model.

The foregoing procedural steps are ysummarized by Figs. 42, 43 and 44, taken vwith the-self-explanatory legends on these views, which'will be referred to in greater detail hereinafter.v

More particularly, either of 'two procedures may be used to effect the required precise orientation of projector l-with respect -to projectorZ v(see Fig. 2) when forward and rear oblique'photographs, respectively, at the same exposure point are yoriented in the projectors. These procedures are as follows: reference being made particularly to Fig. ,2 and alsomore generally at this time to Figs. 42, 43 and 44in-this connection.

In the first method, the objective is to adjust projector 1 so that the -portion'of the projection covering the same lground area-ias-that covered by projector 2 is the exact same size'and shape as the projection from projector 2. The projectionfrom-projector 1 isl viewed on platen 1 of the stereopontometer tobe described in detail hereinafter, and the projection from projector 2 is viewed on platen 2, each of which-has a 4cross mark etchedv or engraved on its surface. These platen surfaces may be opaque, in which case -they may be viewed from above by the use of suitable viewing equipmentv designed therefor, or they may be ground glass surfaces, in which case they may be viewed simultaneously'from beneathv through the binocular system shown diagrammaticallyin Fig. 44, which embraces thejoptical system `of the stereopontometer forming part of the present invention. When identical images in the two projections coincide simultaneously with the cross marks on the two platens in all portions of the -projections common to both when the platens are translated throughout the projections` at a xed. separation, projector 1 is oriented properly with respect to projector 2.

To accomplish this, the platens are positioned first near the `center of the common area which may be regarded as the first position (position No. l) and projector 1 isshifted linearly in the transverse direction indicated by thev arrow bx on Fig. 2; and the separation of the platens is adjusted until the same images in each projection coin'-y cide withv thev cross marks. In making this` orientation,

any want of correspondence in the bxl direction will` cause the cross :to appear `to oat above or below 1h? image. In the binocular view, the two crosses fuse toapwillcause one leg of the cross to appear to oat in space above the other. Then the platens are shifted to the right -or left to the extreme edge of the common area along the center line (which may be considered to be the second position or position No. 2) and adjustment of the swing of projector 1 and the separation of the platens is made to` obtain coincidence of identical images at this position. A check is made at the first position noted above, and the :above steps are repeated until perfect register in the by -direction is obtained along the center line of the common area. The platens then are shifted to the rear of the pro- -jection at the center line of the common area which may ybe regarded as the third position (postion No. 3), and Vprojector 1 is adjusted linearly vertically in the direction indicated by the `arrow bz for obtaining coincidence .at this point. The separation of platens is adjusted if necessary. The platens then are shifted to a position near the -front side ofy the projection along the center line of the common area, which is the fourth position (position No. 4), the disparity in direction by is noted and over-corrected by approximately one-half the amount of disparity of adjusting the tilt of projector 1. The platens are returned to the first position and the entire process is re- '.peated until coincidence is obtained at each of the four positions (positions l, 2, 3 and 4). The platens then are jshifted to one corner of the common projection area, which is the fifth position (position No. 5), and the tip of projector 1 is adjusted to obtain coincidence at this position. This process is repeated until the orientation is found-in which perfect register between the image and the cross marks is observed inall positions of the cornirr'on area-at a fixed separation of the platens. When this orientation is found, projector 1 is in proper orientation for proceeding with the triangulation. It will be under,- lstood that the terms tip, tilt, and swing as applied ,to the projectors are rotary adjustments indicated by curved arrows a', b', and c on Fig. 2, whereas the positions indicated above as positions l, 2, 3, 4, andS are obtained by linear adjustments of the projectors in horizontal and vertical planes. The tive linear positions of adjustments are not shown in the drawings.

The second method for properly orienting the projectors can be applied even when the area common to both exposures is small. In this method, the separation of the projectors and the image-receiving platforms are made identical and the separation of the platens is not altered in the orientation process. After setting the projector and platen separation, the platens are positioned near the center of the common area, which is the first position noted above (position No. l), and projector 1 is adjusted in linear direction by and rotated for tip to obtain coincidence in linear directions by and bx, respectively. The platens then are shifted to the rear of the common area at the center line, which is the second position as noted above (position No. 2), and projector 1 is adjusted in the vertical linear direction bz and rotatably for swing to obtain coincidence at this position. The platens then are shifted to the front side of the projections at the center line, this being the third position (position No. 3), as above, and the want of correspondence in direction by is noted and over-corrected by approximately one-half the amount of disparity. The entire process is repeated until exact register is obtained at all three positions. Projector 1 then is in proper orientation to proceed with the triangulation.

With convergent oblique serial strip photography, to which the present invention relates, the orientation of the left` hand projector of the two-projector instrument which is employed in carrying out the present invention is accomplished with the forward oblique and rearward oblique photographs of the same station in the left and righthand projectors, respectively, and the orientation Qfmthe left hand projector whentaccomplished in no wayV corresponds to the orientation of the right hand projector. It may be noted also that orientation with vertical serial strip photography can be accomplished with the present invention if identical slides are placed in both projectors, and in this respect the present invention is applicable to photogrammetric triangulation with vertical serial strip photography as well as with convergent oblique serial strip photography in a two-projector instrument.

Referring specifically to the stereopontometcr included in the present invention, it may be said to include a stereoscope having a binocular viewing device operatively mounted between a pair of longitudinally adjustable vertically disposed image-receiving screen units or platen units supported in space by a carriage slidably mounted upon a longitudinal track of a second carriage along which it may be moved to provide longitudinal movement to the stereoscope. The second carriage is provided with rollers which ride upon a lateral track provided therefor in a base support along which the second carriage may be moved to provide lateral movement to the stereoscope. The above movements are provided for moving the stereoscope into a position under a pair of projected images where the platen units may be adjusted longitudinally to receive the said projected images for stereoscopic viewing through the stereoscope as has previously been described.

For a detailed description of the stereopontometer, reference is made to the accompanying drawings, Figs. 2a through 41, inclusive. Fig. 2a shows a perspective of the present improved stereopontometer. This instrument includes a base support comprising a pair of spaced longitudinal parallel bars 10, Figs.f2a and 3, which bars support at their ends a pair of transverse parallel angle irons 12 and 14 vertically spaced from the said bars by spacer plates 16. The transverse angle iron 12 carries a bar track 18, Figs. 2a, 3, 4, and 5 adjacent to the upper edge of its vertical flange and spaced therefrom by a plurality of spacer members 2t). The track 18 provides vertical support for the right side of the second carriage having a roller support 22, `hereinafter described, riding on the said track. The remaining transverse angle iron 14 has a pair of plates 24, Figs. 35, 36, and 37 welded to its ends having passages adjacent to their upper ends through which supporting plugs 26 pass into the ends of a tubular track 2S. The plugs 26 are provided with iianges having a plurality of passages through which a corresponding plurality of fastening members 30 may pass into a corresponding plurality of threaded passages of the plates 24 to secure the plugs to the said plates. The tubular track 28 provides vertical support for the left side of the second carriage including a roller support 32 riding on the said tubular track.

The roller support 32, Figs. 2a and 3, is secured to a carriage frame 70 of the second carriage by a plurality of screws 34, Figs. 4 and 5, and includes a plurality of upper and lower rollers 36 having their outer peripheries grooved to fit portions of the track 28. To prevent any play between the rollers 36 and the track 28, the rollers are mounted within eccentrics which as shown in Figs. 25 and 26 consist of eccentric sleeves 38 rotatably seated within the roller support 32 adjacent to passages 40 housing pairs of locking members 42, one of which is threaded to screws 44 and coacts with the other to provide locking means which, when tightened together, engages the eccentric sleeves 38 locking same in a desired position.

To lock the second carriage in a desired position along its tracks 28 and 18 the roller support 32, Fig. 27, has a locking member 46 having a vertical end flange 48 on one end resting on the roller support 32 adjacent to the forward upper roller of the rollers 36 and its opposite end extending over the said roller. The locking member is secured in this position by a screw 50 threadedlyre-- ceived in the support 32 which acts to depress the end- 13 of the locking member into locking engagement Withhe said roller. i

The roller support, Fig. 2a, ofthe second' carriage 22 is secured to the right end of the carriage frame 70 by a plurality of screws 52 and vincludes a pair of rollers 54 which ride along the track 18. AThe rollers 54, Figs. 33 and 34,- are eccentrically mounted adjacent` to the, ends ofthe support 22 in eccentrics including eccentric sleeves S and locking means 57 similar in construction to those described for the rollers of the roller` support 32. The support 22 has a depending web 56 from which an out'- wardly extending flange E18-extends to supporta roller support bearing block 60` securedto the said flange by screws 62 for vertically securing the right end ofthe first carriage securely to the track`18.

The carriage frame 70, Figs. 2g and 4, of the second carriage extends between the carriage roller supports 22 and 32 a-nd includes a horizontal member 72, Fig. 9, having at one side a` depending ilange 74 and at its opposite sideadownwardly and upwardly extending yiauge 76. The upwardly extending portion of flangey 76 is provided on its innerY side with a channell 78, lthe purpose offwhich will be described hereinafter. The horizontal member 72 of the frame member supports a detachable ange member 80 extending upwardly of the horizontal member to a position corresponding to the upwardly extending flange portion of ilange 76 where it is provided with a channel 82'corresponding to channel 78. Closing the ends of the channels -82 land 78 are a plurality of plates 84, Fig. 21, secured to the frame 70Vby screws 86 which support rods 88 so disposedwith respect to the said plates as to lit withinY the channels to furnish tracks for that portion of a plurality of ball bearings 90 extending Within said channels. The relmaining portions of the ball bearings 90 extend within corresponding channels 78 and 82 of a first carriage frame 92 havingY secured ateach end plates 84' carrying corresponding rods 88 which coaet Withrods 8S to form a roller bearing support for the rst carriage along the frame of the second carriage. The roller bearings 90 are spaced along the frame 92 by a spacerV member 94 which as shown in Fig. 1l, consists of a thin plastic pla-te hav,- ing a plurality of spaced passages through which the ball bearings pass. The rst carriage frame 92 is provided with locking means whereby it may be locked in any desired position along the second carriage. Such locking means comprise braking shoes 100 secured to a shoe supporting member 102 by screws 104. This shoe supporting member is provided with top and bottom Wells 106 separated by a web 10S having a passage through which a reduced portion of rod 110 passes. The rod 110 has at its lower end a threaded passage to receive a screw and washer 112 located Within the lower well 106 and carries on its reduced portion a helicalY spring 114 abutting the web S and shoulder 116 of the rod 1,10. The rod 110 is provided with threads 118 adjacent to its reduced portion which are threadedly received through a correspondingly threaded passage 1,20 provided therefor in frame 92 and has at its upper end a knurled knob 122 by means of'which the rod 110 may be turnedto compress the helical spring forcing the braking shoes 100 against the frame 70 of the second carriage.

The first carriage frame 92 carries a plurality of right angle brackets 124, Figs. 3-8, 9 and l0, to which is secured a flat bar 126 having a beveled upper edge also shown in Figs. 9 and 10. The bar 126 has a pair of bearing brackets 123, Figs. 3e8, 9 and l0, secured adjacent to its ends by screws 130 for positioning in space a rod 132 having left hand threads 134 adjacent to one end andfv'right hand threads 136 adjacent to its other end.

, Thefright end of the rod is provided with a knurled head j 1738;ffor apurpose hereinafter described. The threaded jf portions 134 and 136 of the rod 132 pass through nuts "fl 'secured to a pair of image-receiving screens on platen brackets 142 by studs 144 extending from the said brack- 14 ets. The studs 144nt within recesses 1,46 provided .therefor in the nuts 140. which. are Separated from. sec'- ond recesses 148fby thin partitions 150 and are secured in therecesses 146' by screws 152 passing through passages 154' of the nuts '140' through the recesses 148 against thepartitions 1,50 forcing themlagainst the studs V144 thereby takingA out any lateral play which may exist between the studr and recess walls; To provide for any vertical play which may exist between thenuts and brackets, they are interconnected by a spring 156 under tension. 'y

A brackets 15,8, Figs; 1l, l2 and 13, is secured to the bar 1 216l intermediate its Vends by `screws 160 and supports a pin 162' having a disc 164 secured intermedia-te its ends above the said supporting bracket and crank 166 secured; to its upper endi Y A pair 0f binocular eyepiece supporting brackets168g-Figs. y9a and 8, are mounted upon the barv 126 on each side ofthe bracket 158` and areY provided with webs 171 connected to the, disc 164 through' apair of links-170 as shown in Figs. 3 and 8. It is;with lthis arrangement thatthe eyepiece supporting bracketsI 16S maybe spaced by turning the crank 166 for adjusting eyepieces--connected to said brackets. The eyepieceV supporting brackets 168 and the platen unit supportingfbrackets 142, Fig. 14, are similar in construe tion and aremounted upon the bar 126 through a plurality of frictionV rollers 172y riding upon the beveled portion oft-thel bar-126 and lower rollers 174 ridingagainst the said bar. The rollers 17-2 and' 174 are eccentrically mounted-within upper and lowertlanges 176 and 178 respectively ofthebrackets 163-'and"142, the said eccentrics including eccentric sleeves 177 and` locking means 179 similarf-in'lconstruetion to'those previously described for the rollers 361ol'theroller-support 3 2. Theupper flanges 176 of fthe-brackets have-horizontal extensions tlwhich extendover the bar 126 and-have detachabledepending ilangess182'vwhich frictionally engage the back side of thebarf126-compensate fory any clockwise moment which may-beimpar-ted tofthesaid ybrackets -bythe platen units andV binoculars supported by the said brackets. The lowerflanges 1:78 of -these brackets are alsoprovided with retaining llanges 184 which are adapted to engage the barv 126 should a counterclockwise movement beimpartedto-the brackets which would tend to knock the brackets offthe bar. The brackets are provided with openings Yintermediate the anges 176 and 178 through which the rod 132 passes.

Secured to the platen unit brackets 142 are platen units190, Figs. 2a, 9.and14l, comprising image receiving screens 192 made of glassor other sui-table material h'av ing one side ground and provided with suitable reference marks 194, which as shown in Figs. 2a and 3, consists of crossed'lines etched or engraved on the imagereceiving screens. The image-receivingscreens 192 are carried in the upper ends of the platen unit housings 196, Fig. 4l, which house at their lower ends mirrors 198 adapted to reilectthe image of the image-receiving screens. through lenses 200, mounted within the housings 196 adjacent to the binocular eyepieces, through corresponding lenses 20810 mirrors 206 of binocular eyepieces 202 carried by the eyepiece brackets 168.

The binocular eyepieces 202, Figs. 2a, 3, 38 and 39-include housings 204i having the lenses 208 mounted in their sides facing the lenses 200 of the platen units and the mirrors 206 which are so positioned within the said housings.v 20.4 as to direct the image of the'platen unit screens upwardly to second mirrors 211 which are carried adjacent to the upper end of said housings and which are adapted to reflect the images through telescopic eyepiece tubes 210 carried by the housings 204y containing lenses 212 mounted in the lower ends of the eyepiece tubes andeyepieces 214rnounted in their upper ends for binocular viewing of said images.

To accurately determine the distance between the platen units, the stereopontometer is provided with a scalel 

